If there is a third world war then it will be between believers and non believers and what you said is precise but there is a problem there, in Islam too there is fight between Shina and Sunnis and countries like Saudi and UAE have Sunni majority while Iran have a Shia majority and so is the reason they do not jell together and the fight started some 1400 years back and it is highly unlikely they will sort the difference and come together to have a war like that, the rumor is that recently the death of Qassem Soleimani was ordered by the US because he was having talks with the Saudi king to have an alliance to fight against the restrictions they are facing economically.

Once things went off, both were not looking for a war and they backed off, US had some casualties but they played down the situation as everything is fine and if they really wanted to go for a war they would have done that but the consequences will be really high as the possibility of China and Russia siding with Iran and then the allies might even turn on the US in the midst might have reverted the situation.

War is money for some and the common people will get affected if there is a war and hopefully we will not go to those extremes as majority of the countries are spending billions to showcase their military strength and if those kind of money is used for other noble causes the world would have been a better place but the reality is different  .